Recently, R & D of a high-luminance of X-ray generator using laser Compton scattering has been watched. Here, laser Compton scattering is that radiation rays like X-ray are generated at collision of laser beam and electron beam. Laser Compton scattering requires very high pulse-strength of laser and high luminance of electron beam. However, the production of high pulse-strength of laser has been very difficult as described below.
An optical resonator has been known as a tool to accumulate laser. An optical resonator is, in principle, a tool to amplify laser through laser-interference on the resonator mirror surfaces, accordingly, the amplification depends on reflectance of the resonator mirror. As the optical resonator, the Fabry-Perot ring resonator, Michelson interferometer-typed resonator, Fox-Smith interferometer-typed resonator, Mach-Zehnder interferometer-typed resonator, and the like have been known.
Laser-amplification by optical resonators is only made under the condition that a resonator length is equal to an integral multiple of a half wavelength of laser. This is so-called a stationary wave is standing. Because the resonance width of a stationary wave is determined by reflectance of resonator mirrors, the more the reflectance of mirrors becomes high, the more the resonance width becomes narrow, when intend to obtain high gains. For example, if supposing a resonator for obtaining gains of 1,000 times using the mirrors with reflectance of 99.9%, the resonance width is to be 24 kHz or about 1 Å (10−10 m) in resonance position. Consequently, the resonance state will easily disappear by environmental disturbance of mechanical vibrations and thermal expansion. In order to maintain the resonance state, extremely precise feedback-regulation using piezoelectric driving of the resonator mirrors has been required. However, laser magnification achieved by the conventional optical resonators has been limited to about 1,000 times due to limitations of mechanical regulation.
For example, when laser is amplified and accumulated in the conventional optical resonator using a commercially available high-strength of mode-lock laser oscillator (50 W power, 10 psec/pulse pulse-length, 1064 nm wavelength, 150 MHz repetition), the pulse-strength of accumulated laser has been limited to about 100 μJ. The above conventional high-strength of mode-lock laser oscillator is very expensive. Also, there has been a problem that the conventional optical resonator loses most of the accumulation efficiency due to thermal deformation of the conventional resonator mirror because a heat loss of 100 ppm usually arises on the mirror even if laser oscillated by the conventional high-strength of mode-lock laser oscillator may be amplified and accumulated in the optical resonator.
Many techniques to maintain the resonance state of the optical resonator have been presented (Patent Literatures 1-6).
The Patent Literature 1 is the invention relating to the stabilization of a laser oscillator. The Patent Literature discloses the method to stabilize the oscillation frequency of the laser oscillator (1): wherein, the laser being oscillated by the laser oscillator (1) is frequency-modulated by the modulator (3), the frequency-modulated laser is put in the frequency discriminator (6), resonated with a prescribed frequency, taken out from the frequency discriminator (6), and used for the stabilization of the laser oscillator (1). In this method, an error signal is made from the frequency difference between the frequency of the frequency-modulated laser being taken out from the frequency discriminator which is reflecting the resonance state of the frequency discriminator (6) and that of the former frequency-modulated laser, and the frequency of the oscillator (1) is stabilized by using the error signal. That is, the method disclosed in the Patent Literature is well known as the conventional method to stabilize an optical resonator. However, the Patent Literature does not disclose any of a feedback control system to utilize unamplified modulation wave or harmonic.
The Patent Literature 2 is the invention relating to the laser oscillator for optical communication. The Patent Literature discloses the method to stabilize of the optical ring fiber resonator (4): wherein, a single mode laser being oscillated by a single mode laser oscillator (11) is phase-modulated by an electric signal generator (12), the phase-modulated laser is put in an optical loop path of the optical ring fiber resonator (4) comprising plural optical doped ring fibers, an error signal is made from the phase difference between the laser being taken out from the optical loop path and the former single mode laser, the piezoelectric device (9) is driven by the error signal, and an optical loop length of the optical ring fiber resonator (4) is regulated by the piezoelectric device (9). However, there has been unresolved the problem that the pulse power of the optical ring fiber resonator (4) becomes instable because the thermal expansion of the optical doped ring fiber becomes larger with increasing the strength of the accumulated laser in the optical loop path. Although the Patent Literature 2 is an invention relating to the fiber laser oscillator for optical communication, the Patent Literature does not disclose any of a feedback control system to utilize unamplified modulation wave or harmonic.
The Patent Literature 3 is the invention relating to a whispering-gallery mode resonator using the Mach-Zehnder interferometer-typed resonator. The Patent Literature discloses the method to stabilize the Mach-Zehnder interferometer-typed resonator (110), comprising: a beam splitter (113); plural reflection mirrors (114-116); a reference resonator (130) which are arranged in the inner optical path of the resonator; wherein, the laser beam being oscillated by the laser oscillator (101) is split into two beams, one split beam is put in the detection module (120) being arranged in the outside of the optical path via the reference resonator (130), another split beam is directly put in the detection module (120), an error signal is made by the detection module (120), and the reference resonator (130) and/or laser oscillator (101) are tuned by the error signal. Although the Patent Literature discloses the whispering-gallery mode resonator using the Mach-Zehnder interferometer-typed resonator, the Patent Literature does not disclose any of a feedback control system to utilize unamplified modulation wave or harmonic.
The Patent Literature 4 is the invention relating to the Fourier domain mode locking (FDML) operation, a speed-up technique of a high speed wavelength scanning optical fiber oscillator for optical communication. The Patent Literature discloses the feedback control system using operational parameters such as filter tuning, laser gain, polarization, polarization chromaticity, elliptical polarization retardance, and/or dispersion, in order to improve the stability of the Fourier domain mode locking (FDML) operation. Although the Patent Literature discloses the method to stabilize the optical fiber oscillator for optical communication, the Patent Literature does not disclose any of a feedback control system to utilize unamplified modulation wave or harmonic.
The Patent Literature 5 is the invention relating to a method controlling an optical path of an optical resonator. Although the Patent Literature discloses the method to adjust the movement of plural mirrors being arranged in the resonator by adjusting a laser incident angle when the laser is radiated to a lattice, the Patent Literature does not disclose any of a feedback control system to utilize unamplified modulation wave or harmonic.
The Patent Literature 6 is the invention relating to a method to stabilize a wavelength converter to oscillate harmonics. The Patent Literature discloses the wavelength converter oscillating harmonics: wherein, the laser being oscillated by the laser oscillator (12) is radiated to the nonlinear optical material (14) being arranged in the optical resonator (16), the second harmonic is generated thereby, the generated second harmonic is detected by the wave detector (18) being arranged at the back of the mirror (24) of the optical resonator (16), the detected signal is put in the electronic servo circuit to generate an error signal in proportion to the difference in resonance frequency, and the error signal is transmitted to the piezoelectric device (30) being attached to the mirror (26) or the electrodes (32, 34) being attached to the nonlinear optical material (14) to control a resonator length of the optical resonator (16) and/or electric field strength being loaded to the nonlinear optical material (14). Although the Patent Literature discloses the wavelength converter to oscillate the second harmonic and its stabilization method, the Patent Literature does not disclose any of a feedback control system to utilize unamplified modulation wave or harmonic.
The optical resonator system and resonance control method which have been disclosed in the Patent Literatures 1-6 include several difficult problems. As described above, firstly, it has been very difficult to accumulate strong laser in the conventional optical resonator because it has been very difficult to control a resonator length less than 1 Å in resonation position which is required for the laser amplification more than 1,000 times. Secondly, it has been difficult to accumulate strong laser in the conventional resonator because the conventional method has utilized laser strength of amplified laser in the optical resonator as the resonance control signal. That is, the strength of accumulated laser becomes very large when laser is amplified by the conventional method, which induces the occurrence of thermal deformation and laser damage of the resonator mirror due to large heat load on the mirror, and finally generates large resonance fluctuation.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with accompanying drawings.